1. Field of the Invention
This invention relates to a method for sterilizing and/or deactivating adventitious agents such as bacteria, viruses, etc., associated with biological materials, e.g., foods, tissue such as bone, etc., employing a combination of pre-irradiation and irradiation procedures.
2. Description of Related Art
It can be difficult to reduce the bioburden of a biological material, e.g., living tissue, many kinds of proteinaceous substances, drugs, etc., intended for medical/surgical application without negatively affecting the therapeutically useful properties of the material to a significant degree. For example, changes in pH, ionic strength or temperature can result in reversible or irreversible changes in the character of many kinds of biological materials and, consequently, a diminution in their therapeutic effectiveness. Attempts have been made to avoid or minimize irreversible changes to biological materials by sterilization employing ethylene oxide. However, ethylene oxide often reacts with proteins. In addition, because of the known tissue toxicity and the carcinogenic potential of the by-products of ethylene oxide, the United States Food and Drug Administration has set maximum residue limits for ethylene oxide and its major reaction products ethylene glycol and ethylene chlorohydrin.
Unlike ethylene oxide, radiation sterilization has the advantages of high penetrating ability, relatively low chemical reactivity and instantaneous effects without the need to control temperature, pressure, vacuum, or humidity. Radiation sterilization is a very convenient method for sterilizing medical devices, tissue, food, etc., and is widely used in industry. Both dosage levels and its biological effects are well known. It is generally believed that gamma-rays, electron beams, and x-rays as sources of ionizing radiation are equally effective in killing or deactivating microbial organisms. However, radiation can cause damage to the biological materials being sterilized. The damage can result from direct damage caused by the impact of radiation particles with proteins (resulting in broken chemical bonds), or, more commonly, from secondary reactions, usually activated oxygen, e.g., peroxides and oxygen radicals, that are generated by the interaction of the radiation and the material being sterilized. Many of these radicals are oxidizing in nature and do their damage by acquiring electrons from other substances resulting in cross-linking, radical chain reactions and bond breaking.
A variety of methods have been used to reduce or inhibit radiation damage. For example, bioburden is controlled to minimize the radiation dosage required for sterilization. Also, because oxygen is a major source of reactive species formed upon irradiation, removing oxygen from the material to be irradiated can reduce the amount of secondary damage. Oxygen removal is accomplished by evacuating and sealing the package, evacuating and backfilling the package with a less reactive gas and then sealing the package, or by flushing the package with a less reactive gas before sealing. The most frequently used less reactive gas is nitrogen, but others such as argon, etc. have also been used. Oxygen removal, while beneficial, is not completely effective because reactive species can be generated by the action of radiation on water, oxygen containing compounds, etc., that are part of the biological material being sterilized.
Other efforts to minimize the damage to biological materials caused by radiation sterilization have included the use of free-radical scavengers such as, e.g., tocopherol, citric acid, butylated hydroxyanisole, butylated hydroxytoluene, tertiary butylhydroquinone, propyl gallate, ascorbate, and other antioxidants that are “generally recognized as safe” by the Food and Drug Administration. However, these free-radical scavengers may also form undesirable reactive species as a result of the sterilization process.
Lowering the temperature at which sterilization is carried out has also been resorted to. Liquids, when present, are frozen. However, attempts using solutions or other compounds to minimize the effects of free-radical formation during sterilization have had limited success due to the immobility of the compound at the temperatures at which sterilization commonly takes places, e.g., −70° C.
Thus, there remains a need for a method for protecting biological materials against the undesirable effects that frequently occur as a result of the sterilization process.